The invention relates generally to an electrical power transfer arrangement and, more particularly, to a switching arrangement for use with a separately-derived system.
A common electrical power supply system, such as that used in supplying electrical power to a building, includes two hot wires and a neutral wire and delivers dual voltage potentials to the building. For example, a typical dual voltage distribution system has a 240V voltage potential between the two hot wires and a 120V voltage potential between each hot wire and the neutral wire. An electrical distribution panel for use in such a system includes a pair of bus bars, each of which is connected to one of the hot leads, and on which circuit breakers are mounted to distribute the voltage to individual loads. A separate bus is typically provided along one or more sides of the distribution panel for neutral connections. A 240V load is connected across both bus bars while a 120V load is connected to a single bus bar and the neutral bus. Thus, a first portion of the electrical load is supplied via one of the hot wires and a second portion of the electrical load is supplied via the other of the hot wires with the neutral serving as the return wire between both portions of the electrical load.
In today's electrical supply systems, there are occasions when alternate sources of electrical power are necessary or desirable. For example, the capability of switching from a utility supply to a backup generator is extremely important for many businesses, hospitals and industries, as well as residential dwellings. The generator is configured to provide the same voltages as the utility connection, and switching between the power sources is typically performed at a transfer panel. Historically, the pair of hot wires from both the utility supply and the generator were provided as inputs to a first switch. The pair of hot wires output from the switch is connected to the pair of bus bars running beneath the circuit breakers. Whether the neutral wire is switched is dependent on the application requirements. In a non-separately derived system, the neutral wires of both the utility supply and the generator are directly connected to the neutral bar in the distribution panel. In a separately derived system, the neutral wires of the utility supply and the generator are supplied as the inputs to a second switch. The output of the second switch is connected to the neutral bar.
However, switching the neutral wire between the two power supplies in separately derived systems creates the potential for an open neutral event to occur. During an open neutral event, the two hot wires are connected without the neutral wire being connected. This may occur for a brief period during a switching event between power sources in a separately derived system if the switch transferring the hot leads establishes connection prior to the switch transferring the neutral leads. Because both portions of the electrical load share the neutral lead as the return, the resulting effect of the open neutral event is that the two 120V distribution paths are temporarily connected in series rather than in parallel. The 240V voltage potential between the two hot leads is temporarily applied across the two distribution paths in the system until the neutral lead connection is established. Although the period of time during which the 240V voltage potential is present across the devices is brief, it may be sufficient to damage certain devices connected to the distribution system.
Historically, it has been known, therefore, to provide either a single switch or to provide a mechanical interlock between the two switches such that each of the hot leads and the neutral lead for the utility supply and the generator are transferred at the same time. However, such solutions are not without drawbacks. Even though the switch or interlock is intended to simultaneously connect or disconnect each of the hot leads and the neutral lead, manufacturing tolerances and other variations between each throw of a switch creates the potential for delays between connecting or disconnecting the individual throws. The delays may be, for example, up to 10 msec in duration. Because the period of a standard 60 Hertz electrical waveform is 17 msec, such a delay is greater than half of the electrical cycle. If the neutral connection is either opened first or connected last, the delay results in an open neutral event of sufficient length that that either the maximum positive or negative voltage potential occurs at least once before the neutral connection is established.
Thus, it would be desirable to provide an improved switch transfer switch that eliminates the potential for the occurrence of an open neutral event.